DAVID HAHN WAS A COUPLE OF YEARS OUT OF COLLEGE, working gigs as a piano player and dreaming about someday making it to Broadway, when he noticed the first 10 pounds come off his waistline. "I was working out, and it was suddenly like I was doing all the right things. I started giving people diet advice." But the weight kept dropping away. "When you lose 30 pounds and you're not trying that hard, you start to think, something's going on here."

Doctors diagnosed everything from allergies to a tropical infection. Then a CT scan brought the problem into terrifying focus: "I had a giant tumor in the middle of my chest, wrapped around my aorta, my heart, spine, and lungs. I thought, Damn. Nobody gets cancer at 24."

Though he did not know it at the time, Hahn's ability to survive his lymphoma depended on "this little pink flower" from halfway around the world. Doctors started him on 6 months of chemotherapy, and nothing on God's green earth felt remotely natural about it.

The chemotherapy was called ABVD, with the D standing for, he says, "I forget, something nasty." But A and B are both drugs derived directly from bacteria in the natural world; one strain was developed from a soil sample taken from the grounds of a 13th-century Italian castle. The V is vinblastine, from that little pink flower. In the 1950s, researchers from the drug company Eli Lilly began studying the rosy periwinkle; that research led to the development of two drugs, vincristine and vinblastine, both of which gave life back to people with cancers that back then were routinely fatal—leukemia and lymphoma.

Hahn was like a lot of men I talked with in the course of researching this story: They followed healthy diets, they worked out, they thought happy thoughts. Then one day they woke up with night sweats, or found a lump, or developed a cough that wouldn't go away. If they were lucky enough to survive, they tended to thank their doctors or the drug companies. Hardly any of them thought, Whoa, the natural world just saved my life.

Maybe nobody thinks it because we take the natural world for granted. Plants and animals do great things for us all the time, even when we are perfectly healthy. The very air we breathe depends on biodiversity: Prochlorococcus, an ocean-dwelling bacterium that was completely unknown until the 1980s, produces an estimated 20 percent of our oxygen. Plants and other microorganisms do the rest.

And when we're sick? That flower didn't just save Hahn's life. It also turned his career around: "I wanted to make a go at Broadway before cancer, but I didn't have the guts to do it," he says. Then, in chemo, "I was like, You know what, man? If I'm going through all this to save my life, I'm going to have a life that's worthwhile." He's now on Broadway, playing piano for Harry Connick Jr. in On a Clear Day You Can See Forever.

OVER BEERS ONE EVENING NOT LONG AGO, I WAS complaining to a friend about the results of a British study that asked people to define the word "biodiversity." Many thought it was a brand of laundry soap.

Biodiversity refers to the number of different kinds of plants and animals that live in a given area. But the word also has become a kind of shorthand for the health of the entire planet, and the prognosis is not good. Because of human overpopulation, deforestation, overuse of fossil fuels, and other factors, species are now disappearing at a rate last seen 65 million years ago, during the catastrophic era when the dinosaurs went extinct. Only this time, humans are standing in for the dinosaurs.

"Too depressing," my friend said. "If you want men to care about biodiversity, tell them it will help them live longer, avoid baldness, and have better sex." I rolled my eyes.

But I soon found myself accumulating instances of ways the natural world does, in fact, improve men's lives. Men in 11 particular seem to subscribe to the modern delusion that we are a technological superspecies, aloof from the natural world, walled off by science from formerly dread diseases.

In fact, about half the drugs we currently depend on for our modern sense of freedom from disease come directly from the natural world, or they are produced synthetically based on natural models. The antibiotic era, starting during World War II, has changed our lives in ways we can now scarcely imagine. Maybe fungi and bacteria, the sources of many antibiotics, aren't what leap to mind when we imagine the wonders of nature. But without them, you and I might well be dead.

IT'S THE SAME ALMOST ANYWHERE YOU LOOK IN MODERN medicine. For instance, the rate of testicular cancer grew by more than 70 percent between 1975 and 2004. It is now the most common cancer among men ages 15 to 34.

Fortunately, it is also one of the most curable.

Jonny Imerman was 26 and selling commercial real estate in Michigan when he discovered a lump on his left testicle. His oncologist started him on a chemotherapy regimen of three potent drugs: cisplatin, from the heavy metal platinum; bleomycin, an antibiotic from a bacterium; and etoposide, from the roots of a Himalayan plant, the Indian mayapple. It cured him. Prior to the use of this trio of drugs, doctors could cure only about 10 percent of men with testicular cancer that spread to other parts of the body. Now, even with the worst cases, odds of survival are better than 70 percent.

After his recovery, Imerman quit selling real estate and set up a nonprofit group, ImermanAngels.org, which matches cancer victims with mentors of the same age, gender, background, and cancer type. But even with "my head in the middle of all this" cancer stuff, he says, he did not realize that the natural world had saved his life. "You break down the word 'chemotherapy' into 'chemical therapy,' and we just assume these are all chemicals. I did not know that so many of them are derived from natural things."

Nature also deserves much of the credit for the astonishing improvement in our cardiovascular health. Heart disease is still the leading killer of men in the United States. But U.S. heart-disease deaths decreased by 28 percent between 1999 and 2007—a combined gift from a viper and a fungus.

Researchers from Japan and England first discovered the class of drugs called statins in the 1970s. Statins, developed from close relatives of the fungus that gave us penicillin, went on to demonstrate a remarkable ability to knock down levels of bad cholesterol—the low-density lipoproteins, or LDLs, largely responsible for clogging arteries. That meant fewer heart attacks and strokes as statin use grew in the 1990s. British researchers estimated that giving statins to 10 million high-risk people could save 50,000 lives a year.

Our improving cardiovascular health also comes courtesy of the jararaca, a Brazilian pit viper, one of the scariest snakes in South America. Its venom can cause a sudden and steep drop in blood pressure. Brazilian scientists found that the venom acted on angiotensin, a proteinlike compound in the blood known to raise blood pressure. The result was a major new class of drugs: the ACE inhibitors (named for their effect on the angiotensin-converting enzyme), which today are highly effective treatments for hypertension and congestive heart failure.

Ken Cole might not sound like a natural friend of the jararaca; he's a petrochemical lab technician in Houston. But when he caught a cold a few years ago, he made a rare visit to an M.D. The doctor promptly put him on an ACE inhibitor for high blood pressure and a statin for high cholesterol. At age 34 (5'8", 250 pounds), Cole also was diagnosed with type 2 diabetes, and that changed his life: He took up cycling, became a gym rat, and dropped more than 50 pounds. If he is careful, he says, a drug called metformin should keep him from becoming insulin-dependent. That makes him a natural medicine triple hitter: Metformin comes from a weedy plant called goat's rue.

LOOK CLOSELY IN ANY MEDICINE CABINET AND YOU'LL BE struck by just how pervasively nature has shaped our pharmaceutical repertoire. Who would have imagined, for instance, that a Pacific Ocean snail known as Conus magus would give us a painkiller, Prialt, that lacks the addictive-ness of morphine and yet is 1,000 times more potent? Who would have figured that the Pacific yew tree would yield a drug, Taxol, that helps fight cancers of the breast and ovaries and shows promise as a treatment for prostate cancer too?

An estimated 400,000 plant species inhabit the earth, plus untold numbers of insects, marine invertebrates, fungi, and bacteria, each equipped with a unique chemical arsenal of some kind. "We have screened only a small percentage of plants against a small number of possible disease targets," says James S. Miller, Ph.D., vice president for science at the New York Botanical Garden's international plant science center, and even those tests have been limited in scope, focusing mostly on cancers. Millions of insects and other small species have hardly been classified by science, much less tested for their potential usefulness.

So when a forest disappears, what may also be disappearing at the same time is the drug that could keep your hair from falling out or your prostate from swelling. We may be losing the drug that could keep your 8-year-old from dying of a hospital infection or your mother from fading into dementia. You would think scientists and drug companies would be racing to make these discoveries and figure out what's valuable before it vanishes.

One standard shortcut, for instance, is to seek remedies from traditional medicine. That's how scientists found metformin, as well as the drugs derived from the rosy periwinkle, among many others. Even so, as an outsider and a product of the technological world, I didn't expect much when I visited a traditional-medicine market not long ago in Durban, South Africa.

My guide, Jabulani Dhlamini, a practitioner of traditional medicine, held up a sprig of greenery and said, "It's an attraction for the ladies." Pulverize it and put it on your face and, he promised, "any lady who confronts it will agree." (Actually, maybe that claim was no more nonsensical than a typical aftershave commercial.) He also introduced me to so many plant products to cure cancer that it was a wonder the disease ever existed in the first place.

Some years back, Dhlamini showed up at the University of KwaZulu-Natal with a bag of woody chips from the roots of a flowering grassland plant said to cure male impotence. The plant's scientific name is Eriosema kraussianum, but its Zulu name has more poetry: bangalala. University chemists duly isolated five compounds from the plant and put them through standard lab tests, using penile tissue from rabbits. One turned out to be 75 percent as effective as Viagra. Biodiversity comes through again, this time offering us the possibility of better sex. For that matter, Viagra itself has structural similarities to theophylline, a caffeinelike compound found in small amounts in tea. So the future of life on earth and the possibility of sex tonight might be closely related propositions.

RESEARCHERS WHO SEEK CLUES IN TRADITIONAL-MEDICINE markets, like the one in Durban, mostly run into dead ends. But they persist, because now and then the local knowledge accumulated over hundreds of years tips the odds in favor of finding something useful. The hitch is that the only people paying attention these days are underfunded researchers from academia and the government.

Most big drug companies don't rely much anymore on discoveries from the natural world. They depend more on computers and combinatorial chemistry to spin out thousands of closely related variants of a given molecule.

But after 25 years of work and billions of dollars invested, says David Newman, D.Phil., of the National Cancer Institute, combinatorial chemistry has so far produced just one new FDA-approved drug.

Newman, who heads the institute's natural-products branch, isn't suggesting that pharmaceutical companies give up on the technology. But a better way to make the technology work, he argues, is by starting with compounds that are known to be biologically active and structurally similar to those found in the natural world. Then you use combinatorial chemistry to add or subtract traits until you arrive at a drug that meets human needs. "As a discovery tool, combinatorial chemistry is terrible," he says, but "as an optimization tool it is magnificent."

There is one other big reason that men (and drug companies) may want to pay more attention to biodiversity. "Our minds are going to explode over the next 10 years," University of Maryland ecologist Daniel Gruner, Ph.D., told me, "as we learn more about the microbes that are everywhere, including on our bodies, and that keep us healthy all the time." And with the help of genetic analysis, he says, we are beginning to identify them and understand how they work. Instead of the old "war on germs" mentality, researchers are discovering that having the right balance of microbes is essential to well-being, for both ecosystems and individuals.

Obese people, for instance, tend to have a higher proportion of a bacterial group that's highly efficient at extracting nutrients. That may turn out to be why guys with a weight problem can eat the same diet as their skinnier pals and not shed pounds. Understanding biodiversity on that level and learning how to tinker with it may eventually give doctors a subtle new tool for keeping us healthy—and even thin.

Understanding diversity on the larger scale, says Gruner, is also likely to show us how thoroughly we depend on an abundance of plants, animals, and microbes for every aspect of our survival. In a world with so much variety, it's easy to not even notice, or shrug it off, when a species goes extinct. But as the number of species thins out, Gruner suspects that we may find that wetlands no longer purify our water as efficiently, or the oceans do not produce quite as much oxygen, or our farmlands become just a little less fertile.

Gruner has special reason to believe that biodiversity matters. At age 15, he survived leukemia—another win for that little pink flower. For the rest of us, the real question is whether the natural world will still have the answers, and whether the species that could have saved our lives will still be there when it's our turn to look death in the eye.

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